This invention relates to a technology for achieving lower power consumption in an active matrix type liquid crystal display device.
To prevent degradation of a liquid crystal, a matrix type liquid crystal display device for controlling a transmission factor (brightness) of each pixel at an effective value of an applied voltage has to employ so-called xe2x80x9calternationxe2x80x9d in which the polarity of the applied voltage to the liquid crystal is inversed in a predetermined cycle. Since the liquid crystal is made of a dielectric in this instance, charging and discharging of the liquid crystal consume power in the alternation process described above.
U.S. Pat. No. 5,852,426 describes a method of reducing this power consumption. In this method, a switch is provided to change connection of liquid crystal driving electrodes to a liquid crystal driver circuit or to external storage capacitance. The switch selects the external storage capacitance in the first period of one scanning cycle and the liquid crystal driver circuit in the second period. When the external storage capacitance is sufficiently large, the driving voltage can be shifted to a substantial midpoint of the AC amplitude in the first period without consuming power. In consequence, power consumption can be reduced much more than when no means is employed.
U.S. Pat. No. 5,852,426 needs disposition of the storage capacitance outside the liquid crystal driver circuit. Therefore, to employ this method, a new circuit board design such as the arrangement of the storage capacitance and wiring to the storage capacitance becomes necessary, and the number of components increases, too.
It is an object of the present invention to provide a matrix type liquid crystal display device, and a driving method thereof, that can save power consumption resulting from alternation without disposing outside any component.
The present invention for solving the problem described above is based on the concept that a charge stored in a liquid crystal can be initialized to 0 (or substantially 0) when an A electrode and a B electrode sandwiching a liquid crystal between them are temporarily short-circuited at a timing of alternation as shown in FIG. 1. In other words, the sequence having this short-circuit can shift the voltage to a substantial midpoint of alternation amplitude without consuming power. This means that the power-saving effect can be obtained in the same way as in U.S. Pat. No. 5,852,426 without involving the disposition of external components.
The liquid crystal display controller and the liquid crystal display device according to the present invention based on this concept provide switch means for temporarily short-circuiting both of column electrodes and common electrodes sandwiching a liquid crystal in synchronism with the timing of alternation to these electrodes. The present invention includes also means for setting a charge stored in the liquid crystal to or below a predetermined value in synchronism with the timing of alternation. The predetermined value includes 0 (or substantially 0).